For words that have only three letters, there is no 4th letter. One example is with the word “insert”, there aren't any additional letters afterwards and so has to be the word "increase". The word “addict” is during the BIP39 wordlist however you would've employed “addi” for “addict”. If you https://bookmarkforest.com/story19077326/helping-the-others-realize-the-advantages-of-bip39-mnemonic-phrase-tool